The present invention relates to a robot control system which is capable of feedback controlling each articulated joint so as to equalize the detected position to a position command. More specifically, the present invention gives adequate compliance characteristics to the robot control system.
The industrial robots are used in various stages of manufacturing, e.g., in an assembling process or in a machining process, in which the robot is often subjected to an operation for pressing a workpiece on a designated surface.
For example, a workpiece needs to be fitted into a chuck of a lathe. To this end, the robot performs a shifting operation of the workpiece in the following manner. First, the robot moves a workpiece toward the chuck and stops it when the workpiece hits the front surface of the chuck. Next, the robot changes the shifting direction of the workpiece (usually perpendicularly) so that the workpiece slides along the chuck surface under a significant pressing force until the workpiece engages in position with the chuck.
In general, each industrial robot handles or carries various kinds of workpieces. Hence, in the above-described shifting operation of the robot, the stop position of the working end at the moment the workpiece hits the chuck surface varies depending on the size of each workpiece.
Using a visual device or a sensor for detecting the size of each workpiece may be preferable to adjust the stop position of the working end of the robot. However, the cost increases.
The unexamined Japanese patent publication No. 6-39760 discloses an impedance control system for feedback controlling a pressing force of the workpiece acting on the designated surface. However, the robot receives a reaction force which varies depending on the impedance parameter and a position change amount. This makes it difficult to finely adjust the reaction force. Furthermore, when the present position is largely offset from the target position, the reaction force may increase excessively.